The present invention relates generally to brake control systems, and more particularly to an improved wheel deceleration-based antiskid brake controller with adaptive deceleration threshold.
Antiskid brake controllers have been in widespread use for many years. In the simplest sense, an antiskid brake controller compares the speed of a vehicle (e.g., automobile, aircraft, etc.) derived from a wheel speed sensor to the vehicle speed derived from a secondary or reference source. If the wheel is determined to be skidding an excessive amount, then brake pressure applied to the wheel is released and the wheel is allowed to spin back up to the appropriate speed.
A fundamental problem associated with virtually all antiskid brake controllers relates to determining an appropriate amount of skidding. Two types of controllers that are generally known utilize different techniques. The first type of antiskid controller is deceleration-based. In short, the deceleration-based controller differentiates the wheel speed to determine how fast the wheel speed is changing. If the wheel decelerates too quickly, there is said to be excessive skidding and the controller reduces the amount of pressure transmitted to the brakes.
A second type of antiskid controller relies on a model of the mu-slip curve that describes the tire-to-road surface friction characteristics. The difference between the wheel velocity and the vehicle velocity is referred to as the slip velocity. The slip velocity is compared with a predefined set point on the mu-slip curve in order to achieve a desired amount of skidding.
Antiskid controllers that are based on the mu-slip curve are oftentimes computationally complex and require multiple sensors for measuring wheel speed, vehicle speed, etc. Consequently, deceleration-based antiskid controllers may be preferred as a simpler approach. However, deceleration-based antiskid controllers in the past have not been sufficiently adaptive to adjust to varying tire/surface conditions (e.g., wet or icy conditions) insofar as determining an acceptable amount of skidding. Failure to adjust for varying conditions can result in the antiskid controller prematurely releasing brake pressure and/or failing to adequately control excessive skidding.
In view of the aforementioned problems associated with conventional antiskid brake controllers, there is a strong need in the art for an antiskid controller which is adaptive to handle changing tire/surface conditions. In addition, there is a strong need for such an antiskid controller which is not computationally intensive and which does not require multiple sensors, etc.
U.S. Pat. No. 6,178,370 issued to Michael L. Zierolf successfully addressed the above-identified issues. The disclosure of U.S. Pat. No. 6,178,370 is incorporated herein by reference in its entirety. Michael L. Zierolf, also the Inventor of the present invention, has made improvements on the invention disclosed in U.S. Pat. No. 6,178,370 and discloses those improvements below.
The antiskid brake controller of the present invention utilizes measured wheel speed in order to provide brake control for a vehicle, such as an aircraft. The measured wheel speed is differentiated to determine the deceleration of the wheel (deceleration signal xcfx89xe2x80x2). The measured wheel speed is also used to determine a reference wheel speed (wheel speed reference signal (xcfx89ref). The reference wheel speed (wheel speed reference signal xcfx89ref) is the estimated speed of the vehicle. The antiskid controller uses the wheel speed reference signal xcfx89ref to determine a predefined deceleration threshold. The predefined deceleration threshold is the maximum deceleration the wheel can hold without incurring excessive skidding. The antiskid brake controller compares the deceleration signal xcfx89xe2x80x2 to the predefined deceleration threshold. If the wheel decelerates faster than the predefined deceleration threshold, the antiskid brake controller reduces the pilot commanded pressure provided to the brakes by a scaling factor. As the wheel begins to decelerate at a rate less than the predefined deceleration threshold, the pilot commanded pressure is increased until full pilot commanded pressure is otherwise applied. Thus, the antiskid brake controller is capable of operating based only on measured wheel speed. Consequently, additional sensors are not necessary. An optical encoder, for example, located at the wheel itself may generate the measured wheel speed signal.
As mentioned above, the predefined deceleration threshold is a function of the wheel speed reference signal xcfx89ref. The antiskid brake controller selects the predefined deceleration threshold based on the ability of the wheel to hold the pilot commanded brake pressure without excessive skidding. Excessive skidding is considered to be when the wheel decelerates too quickly. Excessive skidding may be determined in a deceleration-based controller by differentiating the wheel speed to determine how fast the wheel speed is changing, for example. The predefined deceleration threshold values are predetermined based on the desired response characteristics of the antiskid brake controller. To reduce computational complexity, the predefined deceleration threshold are contained in a look-up table. The look-up table values are chosen by trial and error, much in the same way that the proportional and integral gains of a traditional Pl controller are determined. That is, a mathematical model of the aircraft and a landing scenario are used to do an evaluation of a particular set of deceleration thresholds DTs. For example, if the simulation shows excessive, repeated skidding at a particular aircraft speed, the deceleration threshold DT is reduced for the corresponding wheel speed reference signal xcfx89ref. Conversely, if braking is slow to respond and efficiency is lost at a particular aircraft speed, the deceleration threshold DT for the corresponding wheel speed reference signal xcfx89ref is increased. In general, higher deceleration thresholds are required for higher wheel speed reference signals xcfx89ref.
The antiskid controller adapts to varying runway conditions. For example, the antiskid controller adjusts the deceleration threshold downward in the event an icy or wet runway surface is encountered. Upon re-encountering a dry or normal runway surface, the antiskid controller increases the deceleration threshold (allows the deceleration threshold to return to normal conditions). The antiskid controller uses the output of the antiskid controller to determine when to use a lower set of gains appropriate for and icy or wet runway surface.
The antiskid brake controller also includes a proportional gain module. Proportional gain is added to the antiskid brake controller to improve its response.
In addition, the antiskid brake controller provides a partial reset of a system integrator in the event of a skid condition. When a skid occurs during a braking operation, pressure to the brake is dumped or released. Further, the antiskid brake controller resets the system integrator to a tunable constant. The tunable constant may be determined a priori. The tunable constant may be seventy-five percent (75%) of the output prior to the occurrence of the skid, for example. Also, resetting the system integrator to a percentage of the output prior to the occurrence of the skid increases response by not having to restart at zero (0) output. This partial reset prevents excessive brake pressure from being applied to the wheel immediately following the skid, thereby prolonging the skidding event.
Additionally, the antiskid controller includes hysteresis in the pressure dump logic. Such hysteresis involves changing the gain in the pressure dump logic between that used to determine a skid event and that used to determine when to reset the controller following a dump condition. By introducing such hysteresis, it is possible to insure that the wheel spins back up to synchronous with the actual vehicle speed before brake pressure is reapplied.
Further, the output of the antiskid controller is used in the brake controller as an upper limit. By using the output of the antiskid controller as an upper limit of the brake controller, the antiskid controller prevents the brake controller from diverging.
According to one particular aspect of the invention, an antiskid brake controller is provided for controlling a braking operation of a wheel of a vehicle based on a wheel speed signal provided by a wheel speed sensor coupled to the wheel. The antiskid brake controller includes a deceleration threshold generator that selects a deceleration threshold based on the ability of the wheel to hold a pilot commanded brake pressure without excessive skidding. The deceleration threshold values of deceleration threshold generator are contained in a look-up table.
According to another aspect of the invention, an antiskid brake controller adaptive to a surface condition is provided for controlling a braking operation of a wheel of a vehicle based on a wheel speed signal provided by a wheel speed sensor coupled to the wheel. The antiskid brake controller includes a reduced integral gain control. An output of the reduced integral gain control increases or decreases the deceleration threshold as a function of the surface condition.
According to another aspect of the invention, an antiskid brake controller is provided for controlling a braking operation of a wheel of a vehicle based on a wheel speed signal provided by a wheel speed sensor coupled to the wheel. The antiskid brake controller includes a proportional gain block to scale the signal output by a proportional amount. The output of the proportional gain block is added to an output of the antiskid brake controller to increase a response time of the antiskid brake controller to allow increased scaling of the brake control Toutput and thus improved antiskid control.
According to another aspect of the invention, an antiskid brake controller is provided for controlling a braking operation of a wheel of a vehicle based on a wheel speed signal provided by a wheel speed sensor coupled to the wheel. The antiskid brake controller includes a reset circuit. The reset circuit resets the output of the antiskid controller to a percentage of the output of the antiskid controller prior to a wheel lock-up condition in order to prevent excessive brake pressure from being applied to a wheel.
According to another aspect of the invention, an antiskid brake controller is provided for controlling a braking operation of a wheel of a vehicle based on a wheel speed signal provided by a wheel speed sensor coupled to the wheel. The antiskid brake controller includes a hysteresis in a pressure xe2x80x9cdumpxe2x80x9d circuit. The hysteresis is used to determine when to reset the antiskid controller after a wheel lock-up event has occurred. Thus, the hysteresis allows the wheel to spin back up to synchronous before resetting the antiskid controller.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects of the invention. However, these aspects are indicative of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.